A technology of forming metals into metallic complexies are an essential technology in fields of plating, analyzing, etc. Conventionally, cyanides, ammonia, organic acids, etc. are used as complexing agents. But, there is no complexing agent which is effective for all metals; and it is difficult to treat waste water with cyanides, which are relatively effective among said agents, due to the harmful effect to human bodies. Thus, a complexing agent, which is capable of effectively complexing many metals without damaging human health, is required.
On the other hand, pollution of underground water by lead is a type of environmental pollution, and thus products containing lead are severely restricted, such that tin-lead solder is replaced by lead-free solder. Thus, plating layers coated with tin-lead solder should be replaced by lead-free solder.
Thus, tin-silver alloy is to be employed instead of tin-lead solder alloy, and Matsushita Electric Company has disclosed the use of tin-silver solder paste (see Nikken Sangyo Press, Feb. 1, 1996). A method of forming a tin-silver solder layer by plating is now required. The tin-silver alloy has anti-sulphurizability, so it is employed instead of silver.
The difference of electrodeposition potential between silver and tin is 900 mV or more as standard oxidation-reduction potential. Cyanide, e.g., potassium cyanide, is included in the plating solution so as to codeposite tin and silver when forming the tin-silver alloy layer. With the cyanide, there are many problems, including polluting waste water, safe work, etc., so that a tin-silver plating solution including no cyanides is required.
An object of the present invention is to provide an aqueous solution for forming metallic complexes which do not contain cyanides; a tin-silver alloy plating solution, wherein, the composition can be optionally changed; and a method of plating in said plating solution.